This invention relates to an apparatus for driving the transversely sealing means of a form-fill-seal packaging machine for forming bags out of an elongated flexible thermoplastic film material by heating it in a direction transverse to its elongation. Form-fill-seal packaging machines of both vertical and horizontal pillow types have been known, and such sealing means are herein referred to summarily as a transverse sealer. It may be called a horizontal heater in the case of a packaging machine of the vertical pillow type, and a vertical heater in the case of a packaging machine of the horizontal pillow type.
Packaging machines of the form-fill-seal type and, in particular, of the so-called vertical pillow type are usually comprised of a shoulder-shaped member disposed above a vertical cylinder which is sometimes referred to as a loading cylinder. A web of belt-like elongated thermoplastic flexible film material, from which bags are to be made, is pulled out from a film supply roll and is wrapped around the loading cylinder by means of the shoulder-shaped member. For this reason, this shoulder-shaped member is sometimes referred to as a former. The side edges of the elongated film material are overlapped and these overlapped parts are thermally sealed as the film material is pulled down along the outer surface of the cylinder such that the film material is made into a tubular form. This sealing process is hereinafter referred to as the vertical sealing. A horizontally elongated heater is provided below the cylinder for a horizontal sealing process whereby the film material now in a tubular form is sealed horizontally into a bag-like form. In the meantime, articles to be packaged are dropped into this newly formed bag through the loading cylinder. Thus structured, packaging machines of the vertical pillow type are considered efficient because the production and filling of the bags can be carried out continuously and concurrently.
The aforementioned horizontal heater for horizontal sealing will be hereinafter referred to as a horizontal sealer and is generally comprised of a pair of horizontally elongated heating means, hereinafter referred to as seal jaws, which are pressed against each other from opposite sides of the tubular thermoplastic material to thereby make a horizontal seal. The methods of horizontal sealing may be broadly divided into the following two categories, the intermittent motion type and the continuous motion type. By a method of the intermittent motion type, the tubular film material to be horizontally sealed is pulled down in an intermittent stop-and-go manner and the seal jaws are moved towards each other and compress the film material therebetween from both sides while the downward motion of the film material is temporarily stopped. By a method of the continuous motion type, by contrast, the film material now in a tubular form is pulled down in a uniformly continuous motion and, in order to secure a finite sealing time, the sealing jaws are caused to also move in a downward direction together with the downward-moving film material. The present invention relates to a driving apparatus of the continuous motion type.
Many driving apparatus for a horizontal sealer have been considered. If the seal jaws are made to undergo circular motions in a .vertical plane, they can make only a point contact with each other at the point where the two circular orbits are tangent to each other. In addition, the sealed surface becomes curved, rather than flat. Japanese Patent Publication Tokkai 62-235006 disclosed a driving apparatus for causing seal jaws to move linearly along the downward path of the film material such that the entire heating surfaces of the seal jaws can remain in contact with the film material for a finite duration of time. As means for providing such a linear motion to the seal jaws, however, generally D-shaped grooves were used to guide the seal jaws supported at end sections of rotary members. If the rotational motion of these rotary members is increased, to reduce the time required for sealing, for example, the force of inertia by the rotary members increases accordingly and an abnormal force tends to develop on the seal jaws where the grooves change their directions. Another disadvantage of this apparatus is that the trajectory of the seal jaws is invariably determined by the guiding grooves. Depending upon the type of the articles to be packaged, it may be desirable to modify the trajectory so as, for example, to be able to give the bag a squeeze in the vertical direction to cause its contents to settle.
In still another aspect of the present invention, it is desirable to be able to adjust the pressure with which the seal jaws are pressed against each other for carrying out the horizontal sealing process. Prior art attempts to make use of a spring for this purpose have not been satisfactory because this tends to increase vibrations especially if the seal jaws are opened and closed at a fast rate. If a motor dedicated to this task is introduced, this affects the overall cost of the system adversely. If use is made of devices including an oil or air pump, on the other hand, there arises the problem of piping and the pressure cannot easily be varied automatically.
The background of the present invention has been described above with reference only to a form-fill-seal packaging machine of the vertical pillow type, but the disadvantages of the kind described above are also experienced with prior art packaging machines of the horizontal pillow type.